The invention relates to the art of arc welding and more particularly to an electric arc welder having a unique controller for performing short circuit electric arc welding.
In the field of electric arc welding of the type especially useful in pipe welding, weld puddle temperature and fluidity are controlled by using a short circuit arc welding process known as STT welding. This technology was developed and patented by The Lincoln Electric Company, and is disclosed in several patents, including Parks U.S. Pat. No. 4,866,247, Stava U.S. Pat. No. 5,148,001 and Stava U.S. Pat. No. 6,051,810. These three patents show STT welding technology in which the present invention is preferably used. Since this technology is well known, incorporation by reference of these patents provides general background information for understanding the implementation of the invention. In practicing STT short circuit welding, a waveform generator produces the precise waveform used in the welding process by creating a series of current pulses the widths of which determine the current flowing in the welding process at precise portions of the cycle. In practicing this type of electric arc welding, as well as other short circuit processes, it is common to use a Power Wave electric arc welder sold by The Lincoln Electric Company. Such inverter based welder is disclosed in Blankenship U.S. Pat. No. 5,278,390. This patent is also incorporated by reference to disclose the general type of welder used to implement the present invention.
Electric arc welding is performed in a variety of processes, such as spray welding, globular welding, and short circuit welding. Irrespective of the process employed, an inverter converts three phase line current to the desired voltage or current. An inverter based power supply for electric arc welding has a digitally based control to produce a desired output current or voltage at the arc welding process. In pipe welding, pulse welding or short circuit welding is preferred for the first weld bead, known as the xe2x80x9croot passxe2x80x9d, that closes the open gap between edges of adjacent workpieces. For best results, the short circuit electric arc welding procedure is implemented by using the patented STT technology. In this technology, a precise current waveform is outputted by the power supply. Spatter reduction is affected by drastically reducing the current just before the shorted metal separates to create a new arc. Thereafter, the current waveform produces a plasma boost for melting the end of the electrode awaiting the next short. The peak current of the plasma boost pulse is gradually reduced in a current tailout until a set background current is reached. Thereafter, the background current is maintained until the molten metal ball shorts against the workpiece causing the next cycle. In this STT electric arc welding process, the current waveform is accurately controlled by a series of current pulses, the widths of which determine the magnitude of the current at any given time in the waveform. By using this technique, the current waveform allows control of the puddle temperature and/or fluidity. Such capability is vital in gap welding applications, such as open root pass in pipe welding. If the puddle is too cold, a lack of metal fusion can result. As a corollary, when the puddle is too hot, a poor internal back bead is possible caused by the weld metal xe2x80x9csucking backxe2x80x9d into the root. In the past, STT short circuit welding has been implemented as a current control process. If the operator wants to change heat in the arc and/or puddle, he must adjust current by the machine setting.
In accordance with the present invention, there is provided an electric arc welder that can function in the STT mode where heat is controlled merely by changing the position of the electrode. As the extension length increases during the arc condition of a short circuit welding process, the heat decreases, and vice versa. The invention is a controller that generates a voltage waveform for the arc portion or condition of a short circuit welding process. Consequently, the power supply is operable in both current and voltage control modes so current control is used when best for the short portion of the welding cycle, while voltage control is used for the plasma portion. Consequently, the current mode of the power supply will use a waveform control that is the same as the STT welder to implement the short circuit condition of the welding cycle. When the molten metal ball on the end of an electrode shorts against the workpiece, a controlled pinch current waveform is implemented. A dv/dt, dr/dt or dt/dt detector determines an impending break or separation of metal from the electrode. Then, a power switch is opened to instantaneously reduce current flow before the electrode separates. This action minimizes spatter. When the short is broken, the arc condition or plasma condition is immediately established. The power supply electronics detects the break in the electrode as an increase in arc voltage. The power supply shifts into the voltage control phase. A voltage waveform is generated to provide a peak voltage that is used to generate a voltage defined power boost pulse. In this voltage mode, current will change due to the desired fixed peak voltage of the voltage waveform. If the voltage control circuitry is precise and the extension length maintained reasonably constant, the welding current will be reasonably constant. The voltage controlled portion of the welding process has a predetermined tailout and a set background voltage to which the voltage transitions to await the next short circuit. By controlling the voltage during the arc condition of the short circuit welding process, the weld puddle temperature and fluidity can be accurately controlled to optimize the welding process and operate in the voltage range of the power supply. This novel technique of using a current waveform during the short circuit condition and a voltage waveform during the arc condition is applicable in any short circuit electric arc welding process. The invention merely involves using a controlled voltage waveform during the arc condition of a short circuit welding process. The short condition of the welding process can be controlled in accordance with standard technology or by the precision current waveform used in the STT technology. The basic aspect of the invention is the current implementation of the short circuit condition with a precision voltage waveform for the arc or plasma condition of the welding process.
In accordance with an aspect of the invention, there is provided an electric arc welder operated to perform a short circuit process with a first waveform controlling a short condition followed by a second waveform controlling the arc condition. The welder comprises a comparator to create an arc signal when the short condition terminates. A controller then shifts the welder from control by the first waveform to control by the second waveform. In response to the creation of the arc signal, the second waveform is a precise voltage waveform wherein the first waveform is a current waveform which may be somewhat conventional. In the broad aspect of the invention, the first waveform is a current controlled waveform and the second waveform is a voltage controlled waveform. Of course, the second waveform is sometimes a wattage control waveform or a joules control waveform. Both of these complex parameters are functions of the arc voltage. Of course, the waveforms are implemented as a series of current pulses that define either the current waveform or the voltage waveform of the invention.
Another aspect of the invention is the provision of a controller for an electric arc welder operated to perform a short circuit process, with a short condition followed by an arc condition. Such controller has a first current control mode during the short condition and a second control mode distinct from the current control mode during the arc condition. This second control mode is preferably voltage tailored by a precise waveform. It has been implemented as a wattage in the precise waveform and a joules control in a precise waveform. The waveform is created in the weld process by a series of current pulses generated or created at a rate exceeding 18 kHz and preferably substantially greater than 20 kHz. In practice, these pulses are created by a pulse width modulator used in a closed loop feedback to control either the current or voltage in accordance with the desired profile outputted from a waveform generator.
Yet a further aspect of the invention is the provision of a method of controlling an electric arc welder operated to perform a short circuit process with a short condition followed by an arc condition. This method comprises employing a first current control mode during the short circuit condition and a second control mode distinct from the current control mode during the arc condition. In practice, this method involves the act of implementing the second control mode as a voltage mode which follows a precise waveform during the arc condition. This provides the advantages previously discussed. The power supply is operated in its voltage range irrespective of the arc resistance.
In accordance with another aspect of the invention, there is provided a digital override switch forcing current control of the power supply during arc condition. This override occurs after the molten metal ball has been formed by voltage control.
The primary object of the present invention is the provision of an electric arc welder, controller for the welder, and method of short circuit welding, wherein the arc condition is accurately controlled by a waveform as a function of voltage. This precision control waveform can be voltage, wattage, which is voltage times current, or joules, which is voltage times current integrated. This voltage function control of the arc condition in a short circuit welding process allows precise heat control of the weld puddle during the welding procedure and operation in the voltage range of the power supply.
Another object of the present invention is the provision of a welder, controller and method, as defined above, which welder, controller and method can be implemented by a single power supply and used for standard short circuit welding and/or STT short circuit welding.
Yet another object of the present invention is the provision of a welder, controller and method, as defined above, which welder, controller and method employs the advantages of current control during the short condition and the advantages of voltage control during the arc condition.
These objects and advantages will become apparent from the following description taken together with the accompanying drawings.